As is known, digital oscillator circuits are very often based upon astable circuits comprising an odd number of logic inverters, connected to one another so as to form a loop, and a plurality of capacitive elements, inserted between the outputs of respective inverters and a reference potential line, for example, ground. Such astable circuits have a natural frequency that depends upon various factors, among which, in particular, the switching voltages of the inverters, the currents supplied by the inverters themselves, and the capacitance of the individual capacitive elements.
Of course, it is of extreme importance that the natural frequency of an oscillator should be equal or at least close to a nominal frequency fixed in the design stage and defined on the basis of particular combinations of the factors listed above.
The fabrication processes currently used are however affected by a margin of imprecision which cannot be eliminated and, consequently, the characteristics of the active and passive components made may differ significantly with respect to the design values. On account of such imprecisions, the switching speed of the individual inverters is different from the one envisaged and consequently also the natural frequency of the oscillator is influenced by the process parameters and can show unacceptable deviations with respect to the nominal frequency.
Currently, it is known in the art to use compensation circuits which enable control of operation of the oscillator so as to modify the natural frequency of oscillation and reduce the sensitivity with respect to the variations of the process parameters.
However, the use of compensation circuits also entails some drawbacks. In fact, even if the imprecisions in the natural frequency of the oscillator caused by the process variations can be in effect reduced, on the other hand, the dependency upon other potentially variable quantities such as supply voltage and temperature increases considerably. Hence, in practice, known compensation circuits introduce factors of instability which render the oscillation frequency in any case far from precise.
In addition, oscillators provided with compensation circuits are rather complex and disadvantageously have large overall dimensions.